1. Field of the Invention
The present invention relates to a momentum exchange system and, more particularly, to a momentum exchange system that is configured to demise on reentry into the Earth's atmosphere.
2. Description of the Related Art
The increase in recent years in the number of spacecraft occupying frequently used orbits has caused growing international concern over orbital debris. Such debris, including retired spacecraft, as well as spacecraft components, has the potential to damage or destroy other spacecraft in these orbits.
Concern over orbital debris has led to international agreements providing guidelines for end-of-mission disposal of spacecraft. Moreover, the U.S. National Aeronautics and Space Administration (NASA) has put in place policy directives and safety standards governing the generation of debris by spacecraft and end-of-mission spacecraft disposal. In order to limit the risk of personal injury or damage to property on Earth, the NASA directives mandate either a controlled reentry, in which the spacecraft is guided to fall into the ocean, or an uncontrolled reentry and demise of the spacecraft.
There are a number of drawbacks to controlled reentry. A controlled reentry requires additional fuel on the spacecraft and may also require additional propulsion systems to properly position the spacecraft to land in a remote location over the ocean. These factors increase the complexity, weight, and cost of the spacecraft.
Another drawback of controlled reentry relates to fault tolerance. Spacecraft system redundancies, particularly in guidance systems, are required to ensure that operators can maintain control of a spacecraft. When redundancy of one or more spacecraft components is lost during a mission, premature controlled reentry may be required. In those situations, spacecraft that are fully capable of achieving their science or other mission goals are lost.
Spacecraft designed for uncontrolled reentry and demise are not subject to the above-described drawbacks. When such spacecraft have reached the end of their mission, they are actively guided back into the atmosphere, or reenter passively through orbital decay, and safely demise with essentially no debris hitting the ground. Demise of a reentering spacecraft is caused by friction between the spacecraft and the atmosphere. The friction applies a combination of deceleration loads on the spacecraft, which act to break the structure apart, and a tremendous amount of heat, which melts or vaporizes the structural components.
There has been some difficulty, however, in designing spacecraft so that all of the components fully demise. In general, a spacecraft component will survive reentry if its melting temperature is sufficiently high, if it is shielded by other components, or if its shape enables it to lose heat fast enough to keep its temperature below the melting point. A particular spacecraft component that is subject to these effects is the momentum exchange system.
In some spacecraft, the momentum exchange system, which comprises a portion of a spacecraft's attitude control system, provides attitude stability to the spacecraft and allows the execution of slewing maneuvers by storing angular momentum and transferring the angular momentum to the spacecraft. The angular momentum is stored and transferred in momentum exchange systems using a rotating flywheel.
Conventional momentum exchange systems comprise a stainless steel or titanium flywheel having a solid web supporting a relatively massive rim. These systems further comprise a drive motor having a stator containing large amounts of iron, and a rotor containing magnets that are buried within thick subcomponents. Finally, the entire assembly is encased in a thick metal enclosure. Thus, conventional momentum exchange systems utilize materials having a high melting point and contain parts that are shielded within the enclosure, thereby limiting the likelihood of demise on reentry.